JP2677989B2 - Pseudo halftone processor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pseudo halftone processing device that binarizes halftone image data to express pseudo halftone. 2. Description of the Related Art A conventional pseudo halftone processing device of this kind is generally
As shown in the figure, the main scanning counter 1 that counts the number of pixels in the main scanning direction, the sub scanning counter 2 that counts the number of lines in the sub scanning direction, and the count values of the main scanning counter 1 and the sub scanning counter 2 respectively. A decoder 3 for inputting and generating a 4 × 4 Bayer type dither pattern m as shown in FIG. 9 according to this input data, a dither pattern m generated by the decoder 3 and a multi-value obtained by a predetermined image reading means. It has the comparator 4 which inputs the image data k of the value and compares the magnitude of both. Then, the dither pattern m is used as a threshold value for binarization, and in the comparator 4, when the multi-valued image data k is equal to or more than the threshold value (k ≧ m), “white”, while less than (k <
In the case of m), "black" was used to perform binarization so as to express pseudo halftones (Takehiko Fukibe, "Image signal processing for FAX and OA", Nikkan Kogyo Shimbun). .
Image data k having, for example, a halftone pattern of 16 gradations as shown in FIG. 11 is input to this apparatus, and this is input according to a dither pattern m shown in FIG.
The dither pattern m in the figure is overlaid. ) Binarized, the 10th
The binarized data as shown in the figure (in the figure, the circles are black pixels, the frame without circles are white pixels) are obtained. However, according to such a configuration, a halftone image can be expressed in a pseudo manner, but as shown in FIG. 10, for example, the contour is blurred and the resolution is lowered, making it difficult to read characters and the like. There was a problem that it would end up. Further, when a halftone image or the like is pseudo-expressed in halftone using binary information, a periodic density change that does not exist in the original image, so-called noise such as moire, is generated, and it becomes difficult to read the image similar to the above. There was a problem. The above problems occur for the following reasons. That is, in the former, the threshold for binarization is, for example, 4 × as shown in FIG.
This occurs because the white / black patterns obtained by binarization are different because they are different for each pixel in the area of No. 4, and the latter is the line of each halftone photograph and the reading device (halftone expression means). It occurs because the densities are different. The present invention has been made in view of the above problems, and is capable of improving the resolution of a binary image such as a character or a drawing without deteriorating the image quality of a halftone image that is pseudo-expressed, and An object of the present invention is to provide a pseudo halftone processing device capable of clearly reproducing a halftone image without generating noise such as moire on a halftone image such as a halftone dot photograph. Means for Solving the Problems In order to achieve the above object, the present invention provides an input unit for inputting multi-tone image information, and an extraction for extracting a pixel of interest and a plurality of reference pixels from the image information input by this input unit. Means for distinguishing the pixel of interest from an edge image when the density difference between the plurality of reference pixels and the pixel of interest exceeds a first set value, and setting the value to either black or white;
When the density difference with respect to the target pixel for all of the plurality of reference pixels exceeds the second set value, the target pixel is identified as a black isolated point image or a white isolated point image, and a predetermined reference value is added. Alternatively, dither preprocessing means for setting the density value of the pixel of interest to a value obtained by performing smoothing processing by subtraction, and image information sequentially output from the dither preprocessing means are dithered by a dither threshold. And dither processing means for performing. According to the present invention, when there is a reference pixel having a density difference between the target pixel and the reference pixel that exceeds the first set value, the present invention identifies the target pixel as a white or black edge, and When the edge is identified, the white or black data is output to the dither processing unit regardless of the pixel data of interest, so that the white or black edge is emphasized and the resolution of the binary image such as a character is improved. On the other hand, when the density difference with respect to the pixel of interest exceeds the second set value for all of the plurality of reference pixels, the pixel of interest is identified as a black isolated point image or a white isolated point image and smoothed with respect to the image data of interest. Since the processed data is output to the dither processing unit, the isolated points are smoothed, and it is possible to remove moire in a halftone image such as a halftone dot photographic image. Embodiment FIG. 1 is a block diagram showing a pseudo intermediate processing device according to an embodiment of the present invention. In FIG. 1, 11 is a pixel data storage unit that sequentially stores image data k quantized to 2 ⁿ gradations, and 12 is a positional relationship as shown in FIG. 2 from the pixel data storage unit 11. Pixel of interest Px, reference pixel Pi (i = 1 to 8)
Pixel data extraction unit that appropriately extracts desired pixel data from among the above data, 13 is the extracted pixel of interest Px, reference pixel
This is a dither pre-processing unit that performs a process to be described later on the pixel of interest Px from the relationship of Pi and outputs it as dither pre-pixel data Px1. In the present embodiment, the dither preprocessing unit 13 determines that the pixel of interest Px and the reference pixel Pi having the positional relationship shown in FIG.
An example is given in which data of (i = 1 to 3) is input as an address. Reference numeral 14 is a dither processing unit that takes in the pre-dither pixel data Px1 and converts it into dither data D. In this embodiment, the pixel data k is the pixel data extraction unit.
It is taken into the pixel data storage unit 11 via 12. The configuration of the pseudo halftone processing device described above will be described in more detail with reference to FIG. The pixel data storage unit 11 shown in FIG. 3 is a RAM 22 that stores pixel data k for two lines.
And a pixel counter 21 for counting the number of pixels in the main scanning direction in synchronization with the pixel data k. The output of the pixel counter 21 is input to the RAM 22 as an address signal of the RAM 22. The RAM 22 receives a timing signal and performs writing and reading, and the read data of the RAM 22 is sequentially input to the pixel data extraction unit 12. The dither processing unit 14 is the main scanning counter 1 shown in FIG.
And a sub-scanning counter 2 and a ROM 27 having the functions of the decoder 3 and the comparator 4 shown in FIG. ROM27 is the dither counter 26
And the output data Rx1 of the dither preprocessing unit 13 are taken in as address data, and the above-mentioned dither data D is written corresponding to these address data. The dither pre-processing unit 13 is provided with a ROM 25 for inputting the data of the target pixel Px and the reference pixel Pi (i = 1 to 3) having the positional relationship as shown in FIG. In this case, as for the data of the reference pixel Pi, the upper 2 bits of the pixel data quantized into 4 bits are stored in the ROM 25.
You are typing in the address. The pixel data extraction unit 12 includes a two-stage flip-flop (F / F) 2 which is synchronized with the timing signal shown in FIG.
3 and 24 are provided, and each data of the target pixel Px and the reference pixels P1, P2, P3 are extracted at the timing shown in FIG. In FIG. 6, n indicates the nth pixel, and the star (*) indicates data that becomes invalid in the processing of the ROM 25. Further, the reference image data is extracted from only the upper 2 bits as described above. The ROM 25 previously writes a coefficient for outputting the pre-dither pixel data Px1 which is the result of the procedure of FIG. 4 to be described later, using the data of each of the pixel of interest Px and the reference pixels P1, P2, P3 as address signals. In this case, the reference pixels P1, P2, P3
Is 2 bit data, the conversion of Pi ← Pi × 4 + 2 is performed to make the data of the reference pixel Pi pseudo 4 bits and then the process of FIG. 4 is performed. The operation of the pseudo halftone processing device configured as described above will be described with reference to FIG. The multi-valued image signal k quantized into 2 ⁿ gradations is stored in the pixel data storage unit 11 via the pixel data extraction unit 12. The pixel data extraction unit 12 extracts the pixel of interest Px data and the reference pixel Pi (i = 1 to 8) data having the positional relationship shown in FIG. 2 as needed, and outputs them to the dither preprocessing unit 13. . The dither pre-processing unit 13 performs a process to be described later on the pixel of interest Px data, and outputs the dither pre-pixel data Rx1 to the dither processing unit 14. The dither preprocessing unit 13 processes the pixel of interest Px according to the processing procedure shown in FIG. First, the presence or absence of edge detection is determined based on the density difference Δi (Δi = Pi−Px) between the pixel of interest Px and the reference pixel Pi (i = 1 to 8) (step (hereinafter referred to as ST) 1). Here, the density difference Δi and the edge detection value TH1 (TH1 ≧
0), if the relation ∃Pi [| Δi | ≧ TH1] ...
On the other hand, if the relational expression is not satisfied while proceeding to 2, it is determined that no edge is detected and the process proceeds to ST5. In ST5, the pixel data Px is held in the data out as data as a candidate for the output pixel data. In ST2, white / black edges are discriminated. That is,
If Pi satisfying the relational expression is satisfied with the relation of Δi-TH1 ..., it is regarded as a white edge and the operation is ST3.
Proceed to. Then, when the relationship of Δi-TH1 ... Is satisfied, it is determined that it is a black edge, and the operation proceeds to ST4. In ST3, OUT ← 15 holds white data, and in ST4, OUT ← 0 holds black data. When either processing of ST3, 4 or 5 is completed, an isolated point is detected (ST6). That is, when the isolated point detection threshold value as the second set value is, for example, TH2 (TH20), the following relationship is satisfied, or ∀Pi [Δi ≧ TH2]. If the relationship is satisfied, the isolated point is set and the operation proceeds to ST7. On the other hand, if the relational expression (1) is not satisfied, the operation proceeds to ST10 with no isolated point, and the data already held in the data out is output as the pre-dither pixel data Px1 (ST10). In this case, if TH2 = 0 is set, the positive / negative of the density difference Δi is directly compared. In ST7, when the relational expression is satisfied, it is determined that a white isolated point is detected, and the operation proceeds to ST8 to perform the smoothing process of Px1 ← Px−α (α0) on the pixel data Px. Here, α is a smoothing coefficient. When the relational expression is satisfied in ST7, the black isolated point is detected and the operation proceeds to ST9, where the pixel data Px is smoothed by Px1 ← Px + α. The data Px1 of the pre-dither pixel Px1 obtained by the above procedure is output to the dither processing unit 14 as pixel data. Then, the dither processing unit 14 calculates binary image data obtained by dithering the fetched pre-dither pixel data Px1 and outputs this as dither data D. In the present embodiment, when the edge detection threshold value is TH1 = 7, the isolated point detection threshold value TH2 = 4, and the smoothing coefficient α = 2, the pseudo halftone processing is performed on the 16 gradation data as shown in FIG. The result is shown in FIG. 7, which is clearer than the conventional processing result shown in FIG. EFFECTS OF THE INVENTION As is clear from the above description, according to the present invention, the edge can be easily emphasized to improve the resolution of a binary image such as a character, and the density change of a halftone image is smooth. It has an effect that noise such as moire can be removed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram showing a pseudo halftone processing device according to an embodiment of the present invention, and FIG. 2 is a position of a pixel of interest and a reference pixel processed by the pseudo halftone processing device. A conceptual diagram showing the relationship,
FIG. 3 is a block diagram showing the configuration of the pseudo halftone processing apparatus of FIG. 1, FIG. 4 is a flowchart showing the processing procedure performed by the dither preprocessing unit of the apparatus of FIG. 3, and FIG. 5 is FIG. 6 is a conceptual diagram showing the positional relationship between a pixel of interest and a reference pixel processed by the device of FIG. 6, FIG. 6 is a time chart of each signal in the device of FIG. 3, and FIG. 7 is an example of processing results by the device of FIG. FIG. 8 is a block diagram showing an example of a conventional pseudo-halftone processing device, FIG. 9 is a schematic diagram showing a Bayer type dither pattern used in the device of FIG. 8, and FIG.
FIG. 11 is a schematic diagram showing an example of the processing result by the apparatus of FIG. 11, and FIG. 11 is a schematic diagram showing pixel data input to each apparatus of FIGS. 3 and 8. 11 …… Pixel data storage unit, 12 …… Pixel data extraction unit, 13
...... Dither preprocessing unit, 14 …… Dither processing unit.

Claims (1)

(57) [Claims] An input unit for inputting multi-tone image information, an extracting unit for extracting a target pixel and a plurality of reference pixels from the image information input by the input unit, and a density difference between the plurality of reference pixels and the target pixel If the first set value is exceeded, the target pixel is identified as an edge image and set to either black or white, while the density difference from the target pixel for all of the plurality of reference pixels is set to the second setting. If the value exceeds the value, the pixel of interest is identified as a black isolated point image or a white isolated point image, and a predetermined reference value is added or subtracted, whereby smoothing processing is performed on the density value of the pixel of interest. Pseudo-halftone processing comprising dither preprocessing means for setting the applied value and dither processing means for performing dither processing on image information sequentially output from the dither preprocessing means by a dither threshold value. Location.